A charge pump is a voltage converter that uses capacitors as energy storage elements to create either higher or lower voltages. Applications of charge pumps are wide spread in power sources and other applications. In particular, a specific application of a charge pump includes generating a bias voltage in a microelectromechanical system (MEMS) microphone.
In general, charge pumps use some form of switching device to control the connection of voltages to the capacitor. For example, a higher voltage can be derived from a lower voltage in two steps. In a first step, a capacitor is connected across the supply and charged to the supply voltage. In a second step, the circuit is reconfigured so that the capacitor is in series with the supply to a load. Ignoring leakage effects, this example method effectively doubles the supply voltage to the load (the sum of the original supply and the capacitor). The pulsing nature of the higher voltage output may be smoothed by the use of an output capacitor as a filter. The control of the switching device is generally driven by a secondary switching circuit.
Charge pumps are often used for transducers, among numerous other examples. Transducers convert signals from one domain to another and are often used in sensors. A common sensor with a transducer that is seen in everyday life is a microphone, a sensor that converts sound waves to electrical signals. A particular family of transducers produced using micromachining techniques are referred to as MEMS transducers, such as a MEMS microphone, for example.
Many MEMS transducers, and particularly MEMS microphones, often use capacitive sensing techniques for measuring a physical phenomenon of interest. In such applications, the capacitance change of the capacitive sensor is converted into a usable voltage using interface circuits. In such capacitive transducer systems, the transducer sensitivity may be related to a bias voltage applied to the transducing element. In such applications, a charge pump may be used to apply a bias voltage to the transducing element.